(a) Technical Field
The present invention relates to an improved rear hub transmission mechanism for use in bicycles.
(b) Description of the Prior Art
At present, light materials like aluminum alloys are gradually replacing iron material in the manufacture of bicycles. But aluminum alloys are too soft. Therefore, manufacturers have to use iron along with aluminum in manufacturing the rear hub transmission mechanisms. The conventional rear hub transmission is as shown in FIG. 1. It mainly comprises a hub body 1 having a through hole 1a in the center thereof for insertion of an axle 2. One end of the hub body 1 is provided with a ball bearing slot 1b for accommodating a ball bearing 3, so that by means of a nut set 3a which screws onto the axle 2, the ball bearing 3 is positioned on the hub body 1. The other end of the through hole 1a of the hub body 1 is internally provided with threads 1c, and a tube 1d projects therefrom. The tube 1d is circumferentially provided with a plurality of recesses 1e. The rear hub transmission of the conventional type further comprises a driving assembly which consists of a mounting 4, a pre-determined number of retaining elements 5, resilient elements 6 and a gear seat 7. The mounting 4 is circumferentially provided with a predetermined number of notches 4a for receiving the retaining elements 5, which are subject to the pressure of the resilient elements 6 at any time. The interior surface of one end of the mounting 4 is circumferentially provided with a plurality of teeth 4b. After the mounting 4 is passed through the axle 2, the teeth 4b at one end of the mounting 4 lock into the recesses 1e of the tube 1d at one end of the hub body 1. The inner surface of the gear seat 7 is also provided with tooth-like ratchet 7a. An inner tube 8 having external threads 8a at one end thereof is passed through a double row ball bearing 9 into a central through hole of the mounting 4 and locked with the internal threads 1c at one end of the through hole 1a of the hub body 1. A sleeve 9a and a nut 9b are used to press the above-mentioned components tightly onto the hub body 1. The outer surface of the gear seat 7 is provided with a gear so that, by means of the assembly described above, when the gear is driven, the hub body 1 may be driven to rotate in a single direction. From the above-mentioned mechanism, it can be seen that when the cyclist pedals the bicycle and the chain causes the gear to turn and brings the gear seat 7 to turn forward, the inner ratchet 7a of the gear seat 7 lock with the retaining elements 5 of the mounting 4 to link-up with the mounting 4, and the teeth 4b in the inner surface of one end of the mounting 4 mesh with the recesses 1b of the tube 1a at one end of the hub body 1 to cause them to move forward. And when the cyclist pedals the bicycle so that the chain engages the gear of the gear seat 7 to turn backward, the ratchet 7a release the retaining elements 5 of the mounting 4 to produce idle rotation.
However, in the conventional methods of using the chain to rotate the gear, the point of application deviates to one side of the hub body 1, and because the recesses 1e of the tube 1d of the hub body 1 and the internal teeth 4b of the mounting 4 are separately molded, there is a relatively considerable difference in their concentricity, which means that there is a greater clearance between the two components so that shock may be easily generated. Besides, since the hub body is formed of soft aluminum material while the mounting 4 is formed of iron material, coupling of the hub body 1 and the mounting 4 is poor. And because the force is received on one side, (which creates torque concentration at one spot), the existing clearance gradually enlarges and the hub body 1 and the mounting 4 become loosened from each other. Therefore, the chain may have a greater swing, which may cause the chain to slip off the bicycle, or the pins of the chain may easily drop off and the chain may easily break if the chain constantly swings to and fro for a long period of time. There are latent dangers, such as poor coupling strength, in this conventional mechanism. After the rear hub transmission of the present invention is assembled, the duration of the bicycle is definitely affected by the poor connection of the mounting and the hub.
In another embodiment of the prior art as shown in FIG. 2, the conventional rear hub transmission mechanism mainly comprises a hub body 1, which has a through hole 1a, the central portion of the hub body 1 is provided with a through hole 1a for insertion of an axle 2. One end of the through hole is provided with a ball bearing slot 1b which receives a ball bearing 3. A nut set 3a is screwably provided on the axle 2 to position the ball bearing 3 on the hub body 1. The inner surface of one end of the through hole 1a of the hub body 1 is provided with a plurality of internal threads 1c. The rear hub transmission mechanism further comprises a driving assembly which consists of a mounting 4, a plurality of rollers and a gear set 6. One end of the mounting 4 extends integrally to form a tube 34a which is externally provided with external threads 41a. The mounting 4 is passed through the axle 2 so that one end thereof is tightened with the internal threads 1c at one end of the through hole 1a of the hub body 1. The inner surface of the gear seat 6 is also provided with tooth-like ratchet 6a so that the rollers 5 may be inserted into the clearances inside the gear seat 6. The circumferential surface of the rollers 5 matches with and contacts a cylindrical body 5a having a suitable resilience. The interior of the rear end of the gear seat 6 is fitted with a double row ball bearing 6a so that by utilizing a sleeve 7 and a nut 7a, the above components are pressed tightly onto the hub body 1. The outer surface of the gear seat 6 is provided with a gear so that by driving the gear, the hub body is driven to rotate unidirectionally.
In the above-mentioned mechanism, the external threads 41a of the tube 4a at one end of the mounting 4 mesh with the internal threads 1c at one end of the hub body. But there is a deviation in their concentricity when their respective threads are formed. As the hub body is made of soft aluminum material while the mounting is made of iron material, their connection is poor; besides, there is the concentration of torque. Consequently, when the mounting 4 rotates forward, there is a tendency that it may screw into the hub body 1, and because internal threads 1c for engaging with the external threads 41a of the mounting 4 are also formed of soft material, the contact surface (also formed of soft material) of the hub body 1 in contact with the mounting 4 is gradually worn away due to constant abrasion and pressure, so that the mounting 4 goes deeper into the internal threads 1c at one end of the through hole 1a of the hub body 1, affecting their coupling precision. Because the internal threads 1c are worn away, the clearance between the internal threads 1c and the external threads 41a becomes enlarged. Hence, the gear swings through a greater angle, causing the chain to slip off. And because the chain constantly swings to and fro, the pins of the chain may also slip out, causing the chain to break. Therefore, there are many latent dangers, and the duration of the bicyle is inevitably shortened because of the poor connection between the hub body and the mounting.